ABSTRACT AND PROJECT SUMMARY This project explores neurobiological mechanisms that mediate sex differences in endogenous pain modulation under orofacial pain conditions. Clinical and psychophysical studies suggest dysfunction of CNS pain regulatory system as a significant factor in clinical pain disorders and that sexually dimorphic endogenous pain modulation may contribute to a greater prevalence of pain conditions in women than men. In this project, we propose that androgen receptor (AR), a nuclear receptor for androgens, and sex-determining region Y (SRY) gene are involved in endogenous pain modulation by regulating opiodergic tone in major brain regions involved in descending pain modulation. The central hypothesis of this project is that AR- and SRY-mediated regulation of opioid receptor (MOR) expression in descending pain modulatory circuits lead to more efficient endogenous pain modulation in males than females. To test this hypothesis, we have successfully developed animal models of diffuse noxious inhibitory control (DNIC), a form of central pain modulation, that clearly show sexually dimorphic responses under non-pathological and craniofacial deep tissue pain conditions. In Aim 1, we will examine whether a reduction AR via RNAi in the rostroventromedulla (RVM), the brain region known to be involved in conditioned pain modulation, impairs DNIC and therefore enhances craniofacial muscle pain in males, and whether overexpression of AR gene in the RVM of females enhances DNIC. In Aim 2, we will determine whether downregulation of SRY with RNAi in the RVM of male rats impairs DNIC, and whether heterologous expression of SRY gene in the RVM of female rats restores DNIC and attenuates craniofacial muscle pain. We will examine whether the manipulation of AR and SRY expression in either sex eliminates sex differences in DNIC and sexually dimorphic expression of MOR in the RVM. Successful achievement of these aims should have broad translational implications for the development of therapeutic strategies that can be customized for men and women, and provide the basis for further investigations into the role of AR and SRY in many other brain regions involved in pain modulation and in identifying additional downstream molecules that are regulated by these factors.